27,891 research outputs found
New Models for X-Ray Synchrotron Radiation from the Remnant of Supernova 1006 AD
Galactic cosmic rays up to energies of around 10^15 eV are assumed to
originate in supernova remnants (SNRs). The shock wave of a young SNR like SN
1006 AD can accelerate electrons to energies greater than 1 TeV, where they can
produce synchrotron radiation in the X-ray band. A new model (SRESC) designed
to model synchrotron X-rays from Type Ia supernovae can constrain values for
the magnetic-field strength and electron scattering properties, with
implications for the acceleration of the unseen ions which dominate the
cosmic-ray energetics. New observations by ASCA, ROSAT, and RXTE have provided
enormously improved data, which now extend to higher X-ray energies. These data
allow much firmer constraints. We will describe model fits to these new data on
SN 1006 AD, emphasizing the physical constraints that can be placed on SNRs and
on the cosmic-ray acceleration process.Comment: 10 pages, 2 figures. to appear in "Cosmic Explosions", proceeding of
the 10th Annual October Astrophysics Conference (ed. S.S. Holt and W. W.
Zhang) LaTex aipproc.st
Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL
Historical Type Ia supernovae are a leading candidate for the source of
positrons observed through their diffuse annihilation emission in the Galaxy.
However, search for annihilation emission from individual Type Ia supernovae
has not been possible before the improved sensitivity of \integral. The total
511 keV annihilation flux from individual SNe Ia, as well as their contribution
to the overall diffuse emission, depends critically on the escape fraction of
positrons produced in Co decays. Late optical light curves suggest that
this fraction may be as high as 5%. We searched for positron annihilation
radiation from the historical Type Ia supernova SN 1006 using the SPI
instrument on \integral. We did not detect significant 511 keV line emission,
with a 3 flux upper limit of 0.59 x 10 ergs cm^-2 s^-1 for \wsim
1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds
to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and
rules out the possibility that Type Ia supernovae produce all of the positrons
in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less
than 10 years. Future observations with \integral will provide stronger
limits on the escape fraction of positrons, the mean positron lifetime, and the
contribution of Type Ia supernovae to the overall positron content of the
Galaxy.Comment: 3 pages, 2 figures, accepted for publication in ApJ
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Apnea, bradycardia and desaturation spells in premature infants: impact of a protocol for the duration of 'spell-free' observation on interprovider variability and readmission rates.
ObjectiveTo study the impact of implementing a protocol to standardize the duration of observation in preterm infants with apnea/bradycardia/desaturation spells before hospital discharge on length of stay (LOS) and readmission rates.Study designA protocol to standardize the duration of in-hospital observation for preterm infants with apnea, bradycardia and desaturation spells who were otherwise ready for discharge was implemented in December 2013. We evaluated the impact of this protocol on the LOS and readmission rates of very low birth weight infants (VLBW). Data on readmission for apnea and an apparent life-threatening event (ALTE) within 30 days of discharge were collected. The pre-implementation epoch (2011 to 2013) was compared to the post-implementation period (2014 to 2016).ResultsThere were 426 and 368 VLBW discharges before and after initiation of the protocol during 2011 to 2013 and 2014 to 2016, respectively. The LOS did not change with protocol implementation (66±42 vs 64±42 days before and after implementation of the protocol, respectively). Interprovider variability on the duration of observation for apneic spells (F-8.8, P=0.04) and bradycardia spells (F-17.4, P<0.001) decreased after implementation of the protocol. The readmission rate for apnea/ALTE after the protocol decreased from 12.1 to 3.4% (P=0.01).ConclusionImplementing an institutional protocol for VLBW infants to determine the duration of apnea/bradycardia/ desaturation spell-free observation period as recommended by the American Academy of Pediatrics clinical report did not prolong the LOS but effectively reduced interprovider variability and readmission rates
On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure
We use the method of Press, Rybicki & Hewitt (1992) to search for time lags
and time leads between different energy bands of the RXTE data for MCG-6-30-15.
We tailor our search in order to probe any reverberation signatures of the
fluorescent iron Kalpha line that is thought to arise from the inner regions of
the black hole accretion disk. In essence, an optimal reconstruction algorithm
is applied to the continuum band (2-4keV) light curve which smoothes out noise
and interpolates across the data gaps. The reconstructed continuum band light
curve can then be folded through trial transfer functions in an attempt to find
lags or leads between the continuum band and the iron line band (5-7keV). We
find reduced fractional variability in the line band. The spectral analysis of
Lee et al. (1999) reveals this to be due to a combination of an apparently
constant iron line flux (at least on timescales of few x 10^4s), and flux
correlated changes in the photon index. We also find no evidence for iron line
reverberation and exclude reverberation delays in the range 0.5-50ksec. This
extends the conclusions of Lee et al. and suggests that the iron line flux
remains constant on timescales as short as 0.5ksec. The large black hole mass
(>10^8Msun) naively suggested by the constancy of the iron line flux is
rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a
mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk
may produce the puzzling spectral variability. Finally, it is found that the
8-15keV band lags the 2-4keV band by 50-100s. This result is used to place
constraints on the size and geometry of the Comptonizing medium responsible for
the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap
Occultation Mapping of the Central Engine in the Active Galaxy MCG -6-30-15
The colossal power output of active galactic nuclei (AGN) is believed to be
fueled by the accretion of matter onto a supermassive black hole. This central
accreting region of AGN has hitherto been spatially unresolved and its
structure therefore unknown. Here we propose that a previously reported `deep
minimum' in the X-ray intensity of the AGN MCG-6-30-15, was due to a unique
X-ray occultation event and that it probes structure of the central engine on
scales < 1e14 cm, or 1.4e-7 arcseconds. The data are consistent with a bright
central source surrounded by a less intense ring, which we identify with the
inner edge of an accretion disk. These may be the first direct measurements of
the spatial structure and geometry of the accreting black-hole system in an
active galaxy.If the ring of X-ray emission is identified with the inner edge
of an accretion disk, upper limits on the BH mass can be derived. Our
occultation interpretation is controversial in the sense that X-ray variability
in AGNs is normally attributed to intrinsic physical changes in the X-ray
emission region, such as disk or coronal instabilities.Comment: 15 pages, 2 Figures. Latex with separate postscript figure files.
Accepted for publication in ApJ Letter
On The Reddening in X-ray Absorbed Seyfert 1 Galaxies
There are several Seyfert galaxies for which there is a discrepancy between
the small column of neutral hydrogen deduced from X-ray observations and the
much greater column derived from the reddening of the optical/UV emission lines
and continuum. The standard paradigm has the dust within the highly ionized gas
which produces O~VII and O~VIII absorption edges (i.e., a ``dusty warm
absorber''). We present an alternative model in which the dust exists in a
component of gas in which hydrogen has been stripped, but which is at too low
an ionization state to possess significant columns of O~VII and O~VIII (i.e, a
``lukewarm absorber''). The lukewarm absorber is at sufficient radial distance
to encompass much of the narrow emission-line region, and thus accounts for the
narrow-line reddening, unlike the dusty warm absorber. We test the model by
using a combination of photoionization models and absorption edge fits to
analyze the combined ROSAT/ASCA dataset for the Seyfert 1.5 galaxy, NGC 3227.
We show that the data are well fit by a combination of the lukewarm absorber
and a more highly ionized component similar to that suggested in earlier
studies. We predict that the lukewarm absorber will produce strong UV
absorption lines of N V, C IV, Si IV and Mg II. Finally, these results
illustrate that singly ionized helium is an important, and often overlooked,
source of opacity in the soft X-ray band (100 - 500 eV).Comment: 17 pages, Latex, includes 1 figure (encapsulated postscript), one
additional table in Latex (landscape format), to appear in the Astrophysical
Journa
The viscosity of Miranda
Voyager 2 images of Miranda revealed a significant history of geological activity. Overlying an apparently ancient cratered terrain are assemblages of concentric ridges, scarps, and dark banded material. The problems that evolutionary thermal and structural modes of Miranda must face, to provide a convincing explanation for such topographic complexity, are examined
Searching for additional heating - [OII] emission in the diffuse ionized gas of NGC891, NGC4631 and NGC3079
We present spectroscopic data of ionized gas in the disk--halo regions of
three edge-on galaxies, NGC 891, NGC 4631 and NGC 3079, covering a wavelength
range from [\ion{O}{2}] 3727\AA to [\ion{S}{2}] 6716.4\AA.
The inclusion of the [\ion{O}{2}] emission provides new constraints on the
properties of the diffuse ionized gas (DIG), in particular, the origin of the
observed spatial variations in the line intensity ratios. We used three
different methods to derive electron temperatures, abundances and ionization
fractions along the slit. The increase in the [\ion{O}{2}]/H line ratio
towards the halo in all three galaxies requires an increase either in electron
temperature or in oxygen abundance. Keeping the oxygen abundance constant
yields the most reasonable results for temperature, abundances, and ionization
fractions. Since a constant oxygen abundance seems to require an increase in
temperature towards the halo, we conclude that gradients in the electron
temperature play a significant role in the observed variations in the optical
line ratios from extraplanar DIG in these three spiral galaxies.Comment: 43 pages, 29 figure
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